The present invention relates to a vent passage forming structure in an earphone and the earphone. More particularly, the present invention relates to a vent passage forming structure in an earphone for forming a vent passage having a smaller sectional area, and the earphone including the structure.
Earphones are sometimes provided with a vent passage that allows ventilation between an inner space and an outer space of a housing. For example, an earplug type earphone (also called as a canal type earphone) in use has its ear piece closely fit to an inner surface of an external auditory canal. This results in a high degree of airtightness between a front space of a diaphragm and an inner space of the external auditory canal. When the earphone is fit to or removed from the ear, there is a possibility of increase or decrease in pressure in the front space, which applies a load to the diaphragm.
There has been known a technique of providing a vent passage through which the front space communicates with the outer space to eliminate the increase and decrease in pressure in the front space to reduce the load applied on the diaphragm. Moreover, there has been known a technique of proving a vent passage that enables ventilation between the front space or the rear space of the diaphragm and the outer space to adjust acoustic characteristics in reproduction, which is not limited to the earplug type earphone. Japanese Patent No. 5575435 (Patent Literature 1) describes an earphone having these vent passages as holes.
A housing of the earphone is usually made of a resin by injection molding with the view of placing importance to costs and mass productivity. A pin for forming a hole in a die for injection molding of the housing has a diameter equal to or more than a prescribed value to give strength to the pin to avoid a break, a damage, and the like in the mass production molding. The prescribed value for the diameter is typically approximately 0.8 mm in diameter. That is, a hole formed on the housing desirably has a diameter approximately equal to or more than 0.8 mm.
On the other hand, in a case of proving the hole as a vent passage, a difference in vent passage sectional areas, which are determined according to the diameters of the holes, quickly reflects to acoustic characteristics in reproduction. As a general tendency, sound pressure in low register goes higher as the vent passage sectional area is made smaller, which results in preferable sounds in reproduction. The vent passage sectional area is an area of a section orthogonal to an extending direction of the vent passage (a direction of flow of an air). The housing of the earphone has a small volume from the first. It is thus necessary to make a hole, as the vent passage, as small in diameter as possible to obtain preferable sounds in reproduction with enhanced sound pressure in low register. However, as previously described, a hole smaller than about 0.8 mm in diameter has a higher possibility of a pin break in mass production molding as the hole is made smaller in diameter, which lowers the productivity. Accordingly, a contrivance to make the vent passage sectional area smaller without lowering the productivity has been desired.